A chemical amplification resist composition is a pattern forming material capable of forming a pattern on a substrate by producing an acid in the exposed area upon irradiation with actinic rays or radiation such as far ultraviolet light and through a reaction using this acid as the catalyst, changing the solubility in a developer between the area irradiated with actinic rays or radiation and the non-irradiated area.
In the case of using a KrF excimer laser as the exposure light source, a resin having small absorption in the region of 248 nm and having a basic skeleton of poly(hydroxystyrene) is predominantly used as the main component, and this is an excellent system capable of forming a good pattern with high sensitivity and high resolution as compared with the conventional naphthoquinonediazide/novolak resin system.
On the other hand, in the case of using a light source of emitting light at a shorter wavelength, for example, in using an ArF excimer laser (193 nm) as the light source, a resist containing a resin having an alicyclic hydrocarbon structure with high transparency has been developed for use with an ArF excimer laser, because the compound having an aromatic group substantially has large absorption in the region of 193 nm. For example, in Patent Document 1 (JP-A-2005-331918 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), Patent Document 2 (JP-A-2004-184637) and Patent Document 3 (JP-A-2003-330192), various properties are improved by introducing a repeating unit having a spacer portion between the main chain and the acid-decomposable group into an alicyclic acid-decomposable repeating unit.
Also, as for the acid generator which is a main constituent component of the chemical amplification-type resist composition, various compounds have been found and a triarylsulfonium salt and an arylalkylsulfonium salt are disclosed, for example, in Patent Document 4 (JP-A-2000-275845), Patent Document 5 (JP-A-10-48814) and Patent Document 6 (JP-A-2005-308969).
A so-called immersion method of filling a high refractive-index liquid (hereinafter sometimes referred to as an “immersion liquid”) between a projection lens and a sample has been conventionally known as a technique for increasing the resolving power in an optical microscope.
As regards the “effect of immersion”, assuming that NA0=sin θ, the resolving power and focal depth in the immersion can be expressed by the following formulae:(Resolving power)=k1·(λ0/n)/NA0(Focal depth)=±k2·(λ0/n)/NA02wherein λ0 is the wavelength of exposure light in air, n is the refractive index of the immersion liquid based on air, and θ is the convergence half-angle of beam.
That is, the effect of immersion is equal to use of an exposure wavelength of 1/n. In other words, in the case of a projection optical system with the same NA, the focal depth can be made n times larger by the immersion. This is effective for all pattern profiles and can be combined with super-resolution techniques such as phase-shift method and modified illumination method which are being studied at present.
Examples of the apparatus where this effect is applied to the transfer of a fine image pattern of a semiconductor device are described in Patent Document 7 (JP-A-57-153433) and Patent Document 8 (JP-A-7-220990).
Recent progress of the immersion exposure technique is reported, for example, in Non-Patent Document 1 (SPIE Proc., 4688, 11 (2002)) and Patent Document 9 (International Publication No. WO2004-077158, pamphlet). In the case of using an ArF excimer laser as the light source, in view of safety on handling as well as transmittance and refractive index at 193 nm, pure water (refractive index at 193 nm: 1.44) is considered to be a most promising immersion liquid. In the case of using an F2 excimer laser as the light source, a fluorine-containing solution is being studied in view of balance between transmittance and refractive index at 157 nm, but those satisfied in terms of environmental safety or refractive index have been not yet found out. Considering the degree of immersion effect and the maturity of resist, the immersion exposure technique is expected to be most soon mounted on an ArF exposure machine.
Also, it is pointed out that when the chemical amplification resist is applied to immersion exposure, the resist layer comes into contact with the immersion liquid at the exposure, as a result, the resist layer deteriorates or a component adversely affecting the immersion liquid bleeds out from the resist layer. Patent Document 10 (International Publication No. WO2004-068242, pamphlet) describes a case where when the resist for ArF exposure is dipped in water before and after exposure, the resist performance is changed, and this is indicated as a problem in the immersion exposure.
As regards the medium filled between a projection lens and a semiconductor substrate, which is used in the immersion exposure, water having a refractive index of 1.44 is employed in view of easy availability and safety and by using an exposure machine having a projection lens with NA of 1.2 to 1.35, pattern formation of a semiconductor device in a design dimension up to the 45 nm generation is considered to be possible.
The generation next to the design dimension of 45 nm is 32 nm, and it is considered that NA of 1.65 is necessary for the pattern formation of a 32 nm-generation semiconductor device and in this case, the medium filled between a projection lens and a semiconductor substrate must have a refractive index of 1.8 or more.
Meanwhile, the construction material of the projection lens having NA of 1.65 is required to have a refractive index of 1.9 or more and LuAg is supposed to be a promising candidate therefor at present, but the problem of large absorption amount of light passed has been not yet solved.
Furthermore, a candidate medium having a refractive index of 1.8 or more has been also not yet found.
For these reasons, a method where pattern formation of a 32 nm-generation semiconductor device is performed by a special pattern forming method using an exposure machine having a projection lens with NA of 1.2 to 1.35 is attracting attention.
Several methods have been proposed for this special pattern forming method, and one of these methods is a double exposure process.
The double exposure process is a method of exposing the same photoresist film twice as described in Patent Document 11 (JP-A-2002-75857), where the pattern in the exposure field is divided into two pattern groups and the exposure is preformed in twice for respective pattern groups divided.
Patent Document 11 indicates that it is necessary in this method to have properties like a two-photon absorption resist, that is, the photosensitivity or solubility in a developer changes in proportion to the square of exposure intensity, but a resist having such properties has been not yet developed.
[Patent Document 1] JP-A-2005-331918
[Patent Document 2] JP-A-2004-184637
[Patent Document 3] JP-A-2003-330192
[Patent Document 4] JP-A-2000-275845
[Patent Document 5] JP-A-10-48814
[Patent Document 6] JP-A-2005-308969
[Patent Document 7] JP-A-57-153433
[Patent Document 8] JP-A-7-220990
[Patent Document 9] International Publication No. WO2004-077158, pamphlet
[Patent Document 10] International Publication No. WO2004-068242, pamphlet
[Patent Document 11] JP-A-2002-75857
[Non-Patent Document 1] Proc. SPIE, Vol. 4688, page 11 (2002)